This invention relates to a hollow fiber membrane type oxygenator and a method for manufacturing the same.
Generally, in open-heart surgery or other surgical operations, blood from the patient is circulated and pumped through an extracorporeal circuit having an oxygenator incorporated therein for the purpose of oxygenation. One type of well-known oxygenators includes hollow fiber membrane type oxygenators which may be subdivided into two classes depending on the type of hollow fiber membrane, that is, homogeneous membranes and porous membranes. The homogeneous membranes cannot be as thin as 100 .mu.m or less because of membrane strength since they are formed from silicone compound. The limited thickness in turn imposes a limitation on the permeation of gases, particularly carbon dioxide gas. A bundle of several ten thousand fibers to be assembled in an oxygenator becomes undesirably larger in size so that a greater volume of priming liquid is required. Additional problems are troublesome manufacture and an increased expense.
Microporous membranes allow gases to pass therethrough as a volume flow because the micropores are substantially greater in size than the molecule of passing gases. A variety of oxygenators have been proposed using microporous membranes, for example, microporous polypropylene membranes. However, the microporous membranes have high permeability to water vapor so that the performance becomes low due to water condensation and plasma can leak out of the membranes after a long term of blood circulation therethrough.
Japanese Patent Publication No. 54-17052 proposes a hollow fiber membrane comprising a hollow fiber substrate having micropores of 10 micrometers or less in diameter extending throughout the side wall wherein a gas-unpermeable thin film of methylhydrogenpolysiloxane is formed on the side wall. However, since the methylhydrogenpolysiloxane film is formed not only in the micropores, but also on the inner and outer surfaces of the hollow fiber substrates, the cavity of the hollow fiber substrates has a correspondingly reduced inner diameter, resulting in deteriorated gas exchangeability. Also, the amount or thickness of methylhydrogenpolysiloxane filled in micropores is correspondingly increased so that the membrane as a whole exhibits reduced percentage permeabilities to gases such as oxygen and carbon dioxide. The polysiloxane-coated membranes have the problem that plasma leakage occurs after a long term operation as an oxygenator although they can be sufficiently employed in aqualungs or similar equipment.
In order to overcome these problems, the applicant proposed in U.S. patent application Ser. No. 597,440 an improved oxygenator using hollow fiber-silicone composite membranes comprising hollow fibers having micropores filled with a silicone oil without forming a silicone oil layer on the side wall. This oxygenator is manufactured by prefabricating an oxygenator module having untreated hollow fibers incorporated, impregnating the hollow fibers with a silicone oil solution, removing the solution, and then passing a mixture of a solvent and a non-solvent for the silicone oil to thereby remove silicone oil deposits on the hollow fiber substrate side wall, thereby filling only the micropores with the silicone oil. The silicone oil-filled membranes are improved in plasma leakage, but have the possibility that the silicone oil will flow into the contacting blood.